1. Field of the Invention
The present invention relates to exposure apparatuses and device manufacturing methods.
2. Description of the Related Art
For manufacturing devices such as semiconductor devices and liquid crystal devices by photolithography, projection exposure apparatuses that transfer patterns formed on reticles onto wafers by exposure are used. Generally, a projection exposure apparatus needs to accurately project a pattern of a reticle onto a wafer at a predetermined magnification. Therefore, a projection exposure apparatus requires a projection optical system with a high image forming capability and low aberration. Due to a demand for miniaturization of semiconductor devices in recent years, such requirement is increasing. For this reason, there is a demand for the ability to measure the wavefront aberration of a projection optical system with high accuracy in a state where the projection optical system is installed on the exposure apparatus body.
To fulfill this demand, there has been proposed an exposure apparatus equipped with an interferometer used for measuring wavefront aberration of a projection optical system. Examples of such an exposure apparatus are disclosed in Japanese Patent Laid-Open Nos. 2000-277412, 2004-273748, and 2006-086344. As an interferometer, a line diffraction interferometer (which will be referred to as an “LDI” hereinafter) or a shearing interferometer can be used.
An LDI will be described below with reference to FIG. 9. FIG. 9 is a schematic diagram of an LDI equipped in an exposure apparatus. A reticle 1 disposed at an object-plane side of a projection optical system 5 or a reticle reference plate 4 provided on a reticle stage has two parallel slit patterns (slit A and slit B) arranged adjacent to each other as shown in FIG. 10A. At least one of the slits, i.e. slit A in this case, is given a width that is set smaller than or equal to the resolution at the object-plane side of the projection optical system 5. In other words, if the numerical aperture at the object-plane side of the projection optical system 5 is represented by na and the wavelength of exposure light is represented by λ, a width w of the slit A is set so as to satisfy the condition: w≦0.5×λ/na. When an illuminator 6 illuminates the slits A and B with exposure light, the light released from the slit A will have a wavefront with no aberration with respect to the lateral direction of the slit A. On the other hand, the slit B may have the same line width as the slit A or may have a line width greater than that of the slit A. If the line width of the slit B is set greater than the resolution, the light released from the slit B will have a wavefront affected by aberration of an illumination optical system of the illuminator 6. The slits are given a length that can be considered that the aberration of the projection optical system 5 is the same between the two, or in other words, a length that is shorter than an isoplanatic area. The two slits are separated from each other by a distance shorter than the isoplanatic area.
Since the light rays released from the slits A and B pass through the projection optical system 5, the wavefronts thereof are affected by the aberration of the projection optical system 5. The images of the slits A and B are formed on a wafer reference plate 10 disposed on an image plane of the projection optical system 5. The wafer reference plate 10 is provided with a slit C and a slit D. The slit C is disposed at a position corresponding to the image of the slit A, whereas the slit D is disposed at a position corresponding to the image of the slit B. The slit D is given a width that is set smaller than or equal to the resolution at an image-plane side of the projection optical system 5. In other words, if the numerical aperture at the image-plane side of the projection optical system 5 is represented by NA and the wavelength is represented by λ, a width W of the slit D is set so as to satisfy the condition: W≦0.5×λ/NA. A wavefront of light focused on the slit D is affected by the aberration of the projection optical system 5 and the aberration of the illuminator 6 depending on the line width of the slit B. However, since the light focused on the slit D passes through the slit D, the light is shaped to have a wavefront with no aberration with respect to the lateral direction of the slit D. On the other hand, the line width of the slit C is greater than the resolution of the projection optical system 5 and is preferably greater than or equal to ten times the wavelength of exposure light and lower than or equal to hundred times the wavelength of exposure light. A wavefront of light focused on the slit C is affected only by the aberration of the projection optical system 5 with respect to the lateral direction of the slit C. Since the line width of the slit C is sufficiently large, the light released from the slit C will have a wavefront affected only by the aberration of the projection optical system 5.
The light released from the slit C and the light released from the slit D are caused to interfere with each other, thereby forming interference fringes. These interference fringes are picked up by an image pickup element 31 such as a CCD sensor, thereby obtaining a wavefront of the projection optical system 5 (i.e. a wavefront in a first direction) with a proper correlation in a direction orthogonal to the longitudinal direction of the slits. Using slits A′ to D′ extending orthogonally to the longitudinal direction of the slits A to D as shown in FIG. 10B, a wavefront in a second direction can be similarly obtained. The wavefront (synthesized wavefront) of the projection optical system 5 can be determined by using the wavefronts (first-order wavefronts) in the first and second directions.
A method for determining a wavefront of the projection optical system 5 from two first-order wavefronts will be described with reference to FIGS. 11A to 11C. FIG. 11A shows a first-order wavefront E with a proper correlation in the Y-axis direction. FIG. 11B shows a first-order wavefront F with a proper correlation in the X-axis direction. FIG. 11C shows a wavefront G of the projection optical system 5. The phase correlation of lines parallel to the Y axis in the wavefront E is equal to the phase correlation of lines parallel to the Y axis in the wavefront G. Moreover, the phase correlation of lines parallel to the X axis in the wavefront F is equal to the phase correlation of lines parallel to the X axis in the wavefront G. Based on these relationships, a phase of an arbitrary point G(x,y) on the wavefront G is determined as phase-change amounts along two arrows in FIG. 11C, and is expressed as follows:G(x,y)=E(0,y)−E(0,0)+F(x,y)−F(0,y)Consequently, the wavefront G of the projection optical system 5 can be obtained from the two first-order wavefronts E and F.
A shearing interferometer will be described below with reference to FIG. 12. FIG. 12 is a schematic diagram of a lateral shearing interferometer whose shear direction extends in a lateral direction (which is orthogonal to the optical axis). The interferometer includes a diffraction grating holding member 37 which supports diffraction gratings 36a and 36b. In this shearing interferometer, a spherical wave released from a spot on the object plane of the projection optical system 5 is made to enter the projection optical system 5. Specifically, the reticle 1 or the reticle reference plate 4 on the reticle stage is provided with a pin hole 35 as a first mark, and the illuminator 6 illuminates the pin hole 35 to generate this spherical wave. The light transmitted through the pin hole 35 passes through the projection optical system 5, which is a subject for measurement, and is split into a plurality of diffracted light rays by diffraction grating 36a disposed between the projection optical system 5 and the image plane of the projection optical system 5. The wavefronts of these diffracted light rays all have a shape that is the same as the shape of the wavefront of the light prior to being split by the diffraction grating 36a. Of the plurality of diffracted light rays, an order selection window 38a transmits only two diffracted light rays used for measurement. The two diffracted light rays transmitted through the order selection window 38a, formed in wafer reference plate 39, form interference fringes that are arranged in the same direction as the direction in which the gratings of the diffraction grating 36a are arranged on an image pickup element 40. A first-order wavefront in the first direction is calculated from these interference fringes. In this case, the order selection window 38a used transmits positive (+) first order and negative (−) first order diffracted light rays but blocks zero-th order diffracted light. The image pickup area of the image pickup element 40 is disposed at a conjugate position with respect to the pupil plane of the projection optical system 5.
Subsequently, the order selection window 38a and the diffraction grating 36a are switched respectively to an order selection window 38b and a diffraction grating 36b having gratings arranged in a direction orthogonal to the direction in which the gratings of the diffraction grating 36a are arranged. Then, a first-order wavefront in the second direction is similarly calculated. From these two wavefronts, the wavefront of the projection optical system 5 is calculated.
In wavefront measurement using an LDI, first-order wavefronts of a projection optical system with proper correlations are determined in two directions and are synthesized in order to calculate the wavefront of the projection optical system. If the positions (Z-axis positions) of slit patterns differ from each other in the optical-axis direction of the projection optical system at the time of the measurement of a first-order wavefront in the first direction and the measurement of a first-order wavefront in the second direction, there will be a significant error.
When using two first-order wavefronts whose first direction and second direction are orthogonal to each other, there is an error in a 2θ component having a protrusion (or a depression) for the first direction and a depression (or a protrusion) for the second direction. In this case, an amount of error E2θ in the 2θ component is expressed as follows:E2θ=dz×(1−sqrt(1−NA2)/2×λ)where dz represents a Z-axis-positional difference in the slit patterns at the time of the measurement of a first-order wavefront in the first direction and the measurement of a first-order wavefront in the second direction, λ represents the wavelength of exposure light, and NA represents the numerical aperture of the projection optical system. In an exposure apparatus where λ=193 nm and NA=0.9, when dz=1 nm, E2θ is 1.5 mλ. In other words, even if there is an error of 1 nm in the Z-axis direction, it is still considered that an error of 1.5 mλ will be generated, which is a significant error factor in the LDI measurement. In a projection optical system with a higher NA, the amount of error will be even greater.
This problem is seen not only in a wavefront measurement method that employs an LDI, but can also be seen in a wavefront measurement method where a synthesized wavefront is calculated from first-order wavefronts in two or more directions (such as a wavefront measurement method that employs the aforementioned shearing interferometer).
Accordingly, it would be advantageous to provide an exposure apparatus equipped with a projection optical system that projects a pattern of an original onto a substrate, and method of operating the apparatus, which is not subject to the aforementioned disadvantages described above.